The invention relates to a method for operating a wind energy plant and to a wind energy plant having a rotor with at least one rotor blade which has an adjustable pitch angle, a generator coupled to the rotor, and a regulation means which, if there is a deviation of the rotation speed of the generator or rotor from a rotation speed set point, adjusts the pitch angle of the at least one rotor blade such that the rotation speed set point is reached again.
Fault of a grid which is fed by the wind energy plant may cause a sudden load shedding, wherein the generator torque or the generator power of the wind energy plant falls off to zero within a very short time period. Moreover, it is also possible to reduce an electrical quantity provided by the plant by means of a generator and fed into a grid to a predetermined value in a controlled manner and in a short time by reduction of a preset value for the generator, such as a preset generator torque or the preset generator power, respectively, for example by means of a corresponding control means of the plant. In this context, in a short time means a time period of less than one minute, in particular less than half a minute.
With increasing importance of regenerative energy sources, energy supply companies more and more make the same technical demands on wind energy plants or wind parks, respectively, as they do on conventional power plants. An example is the demand for a contribution of wind energy plants to a fast reduction of excess power in the grid. To this end, the energy supply company sends a respective signal predetermining a defined power level. Such a signal can be independent of other preset values, for example active power set points. The energy supply companies more and more demand a very fast reduction of the electrical energy of the plant fed into the grid. In a couple of countries, time periods of a few seconds, for example five seconds, are discussed or already required for a reduction of load to a value of for example 20% of the capacity of the grid connection.
In order to meet these rigorous timing requirements, the electrical quantity provided must be reduced by means of a very fast reduction of the generator torque or generator power, respectively. Through this reduction of the counter load generated by the generator, however, the rotor of the wind energy plant is accelerated and rotation speeds occur which cause the regulation means to interact by increasing the pitch angle. The pitch angle is a constructive quantity, namely the angle between the rotor plane and the chord line of the rotor blade. As soon as the regulation means notices a deviation of the rotation speed of the rotor or generator from the rotation speed set point, it reacts by changing the pitch angle. However, the regulation means is designed with regard to the usual wind turbulences. With regard to a sudden load shedding, the regulation means reacts too slowly. This can lead to detrimental excess rotation speeds of the rotor or the generator, respectively. To avoid this, the generator torque or the generator power, respectively, can only be reduced with a limited speed. For this reason, it is difficult to fulfill the strict requirements of the energy supply companies with current systems.
In particular for wind parks, an additional problem is that the request for a reduction of the overall active power provided must be allocated to the individual wind energy plants in an intelligent manner. Receiving, evaluating and transmitting the individual set point signals to the plants by means of the wind park controller takes a certain time, which is then no longer available for a reduction of the load by the plants. The aforementioned problem is therefore getting worse for plants in wind parks.
With known systems, it is not possible to change the active power of a wind energy plant to 20% of the rated power within a few seconds, for example within five seconds. In the alternative, the electrical quantity provided can be reduced by disconnecting the plant from the grid, that is, by a shutdown, but this is often undesirable. In particular, a shutdown of the plant is acceptable only under certain exceptional conditions.
Based on the prior art described, the invention was therefore based on the problem to provide a method and a wind energy plant that allow for a fast load reduction to a new stable operating point without disconnection from the grid.